This invention relates generally to large combined cycle power systems having a steam turbine and gas turbine, together driving an electrical generator as the load, and more particularly to improvements in a tandem arrangement of gas turbine, steam turbine and generator on a single shaft system of rigidly coupled rotor members and having a single thrust bearing for the rotor system.
Many arrangements for gas turbines and steam turbines in a combined cycle have been proposed. A combined cycle is an integrated thermal cycle, wherein the hot exhaust gas from a combustion gas turbine contributes heat energy to partially or wholly generate the steam used in the steam turbine. Combined cycle power plants have been built with the gas turbine, steam turbine and generator rotors connected in tandem by flexible or by solidly bolted rigid couplings. If the rotor members in the single shaft system are rigidly coupled, only one thrust bearing can be employed, since the shaft system expands or grows in an axial direction due to thermal expansion from the thrust bearing.
A proposal for a single shaft combined cycle having gas turbine, single-flow non-reheat steam turbine and generator connected in tandem in the order aforesaid on a single shaft system with rigidly coupled rotor members and a single thrust bearing is disclosed in Japanese patent publication No. 62-38525, published May 6, 1983. In this publication, the single thrust bearing is located between the gas turbine and the steam turbine. The single-flow steam turbine section is arranged in flow opposition to the gas turbine in order to create thrust in opposition to that of the gas turbine, and thereby reduce the size of the thrust bearing. However, no provisions are shown for support of the members on the foundation or to provide for thermal expansion.
Gas turbines have now been developed that have substantially higher power ratings and higher exhaust temperatures than previously available. Because of this, higher rated steam turbines are required and a reheat steam cycle is economically attractive for combined cycle applications. A conventional reheat steam turbine requires at least two casings which are normally arranged to reduce the unbalanced thrust. A typical arrangement for a two-casing reheat turbine having a single flowed low pressure section has three bearing standards, the center bearing standard supporting one end of each of the casings and also containing a thrust bearing. The two casings are keyed to the foundation at only one point, preferably the center of the exhaust hood connection in the low pressure section. The center bearing standard with the thrust bearing is arranged to move relative to the foundation. The two casings supported by the center bearing standard are also keyed to it so that the casings move together with the bearing standard, while the shaft and both casings grow thermally in opposite directions from one another relative to the center bearing standard. While this is an ideal arrangement for a reheat steam turbine, it is not possible to simply substitute a conventional two casing reheat steam turbine design into a single shaft combined cycle of the type described above, since a two casing reheat steam turbine has a moving thrust bearing between the casings. Keying of the thrust bearing to the foundation on either outside end of the two casings or keying the center thrust bearing standard to the foundation between the casings is not normally considered because this would create large differential thermal expansion between rotating and non-rotating members and result in unacceptable efficiency losses.
Accordingly, one object of the present invention is to provide an improved combined cycle plant having a gas turbine, as steam turbine and a generator arranged in tandem in the order aforesaid on a single shaft system with a single thrust bearing, while attaining the high efficiency of a conventional steam turbine design.
A further object of the invention is to provide an improved reheat steam turbine foundation support and anchor point arrangement which is especially adapted for such a single shaft combined cycle plant.